Can anyone explain this function?

Question

It is a simple program to find out if an integer is a power of 4, but I couldn't understand this section when we take integer 255 & 256:

 $x = $n;
              while ($x % 4 == 0) {
              $x /= 4;
             }

            if($x == 1)

Can someone explain it to me?

<?php
    function is_Power_of_four($n)
    {
          $x = $n;
          while ($x % 4 == 0) {
          $x /= 4;
         }

        if($x == 1)
        {
            return "$n is power of 4";
        }
        else
        {
            return "$n is not power of 4";
        }

    }
    print_r(is_Power_of_four(4)."\n");
    print_r(is_Power_of_four(255)."\n");
    print_r(is_Power_of_four(256)."\n");
  ?>

Look up [bitwise operators](http://php.net/manual/en/language.operators.bitwise.php). — Jonnix, Aug 22 '17 at 12:43
For me that duplicate is too narrow. This question is more about a particular use of the bitwise AND operator. — Bathsheba, Aug 22 '17 at 12:48
I know Bitwise operator But it was used for find if a integer is power of 2 — Legend, Aug 22 '17 at 12:50

Bathsheba · Accepted Answer · 2017-08-22T13:37:57.733

1

It's idiomatic in older languages like C.

A number x greater than 1 that is a power of two will have an binary representation 1 0 ... where ... is any number of zeros.

One less than a number that's a power of two (x - 1, say) will have the binary representation 1 ... where ... is any number of ones. Furthermore it will have one fewer binary digit than x. It's also obvious that a subtraction of one from a power of 2 flips all the bits in that original number: no other number has this property.

& is the bitwise AND operator. Reference http://php.net/manual/en/language.operators.bitwise.php

Hence x & (x - 1) will be 0 if and only if x is an exact power of 2.

edited Aug 22 '17 at 13:37

answered Aug 22 '17 at 12:44

Bathsheba

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Can you be more specific from line 3 ? I didn't understand the line from - "One less than a number That's a ........... power of two (x-1, say) will have....... – Legend Aug 22 '17 at 13:03
1

E.g. take 16. That is 0b10000. 15 is 0b1111. 0b10000 & 0b1111 is 0. – Bathsheba Aug 22 '17 at 13:29

Can anyone explain this function?

1 Answers1